[media] DocBook/media/v4l: il_* fields always 0 for progressive formats

Clarify that the il_vfrontporch, il_vsync and il_vbackporch fields must
always be 0 for progressive formats.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>